Process of preparing target screens for orthicon tubes



g- 1957 J. E. CLEMENS ET AL 2,804,403

PROCESS OF PREPARING TARGET SCREENS FOR ORTHICON TUBES Filed Jan. 15, 1954 I E IN V EN TOR.

Jw/A E. a 415; By Bill a Jflil/VTfl/YE Unitd States Patent PROCESS OF PREPARING TARGET SCREENS FOR ORTHICON TUBES John E. Clemens, Greene County, and Ben B. Johnstone, Dayton, Ohio, assignors of one-third to Chester Tietig, Park Hills, Ky.

Application January 15, 1954, Serial No. 404,309

2 Claims. (Cl. 117210) This invention relates to an improved target plate for Orthicon and related electronic tubes. These tubes are at present in use as the seeing tubes of television cameras, guided missiles and other electronic devices. The general purpose of this invention is to improve these tubes by improving their target plates. Such tube improvement has produced the signals by which it is possible to see distant terrestrial objects illuminated in nature solely by starlight with the same clarity and resolution as the average current television broadcast presentation.

The present weaknesses of conventional Orthicon tubes, which this invention is intended to overcome, are those which lead to poor contrast, necessitate high energy levels of illumination and result in a general lack of sensitivity. We have traced the greater part of these troubles to the characteristics of the conventional target plate, which does not have enough storage time because of low resistance between the active elements of which it is composed. The low resistance, which allows leakage of electrical potential to take place to neighboring active elements with the production of the bad effects above mentioned and a bright object with a background having a low light level creates halation. The undesired halation results from the comparatively slow bleeding out of high voltage areas produced by the bright object which are not completely neutralized during each scan, into the adjacent areas.

One object of the present invention is, therefore, to provide a target plate having such a high resistance between discrete areas of active elements so that the ability to leak to other areas is inhibited.

Another object is to provide a target plate of discrete areas of active materials of such a low resistance that the scan rate may be increased due to faster and better neutralization of charge. Extreme speed of scan is for example, greatly desired in the sensing systems of guided missiles.

Reference is now made to the accompanying drawings in which the first five views show end views of the wires of a screen of screens. Such presentation most easily illustrates the steps by which the target is produced. It is to be understood that the completed target comprises two screens, the rear or lower one of which is coated with active material while the front or top one is a bare metallic screen approximating the active one in all dimensions so as to produce a necessary capacity efiect between the two. Such capacitance is conventional in present targets and we make no claim to it per se. While we do not limit ourselves to any particular wire size, meshes per inch or target dimensions or metals, we prefer to use a target comprising two pieces of stainless steel gauze approximately one inch by one inch made of one mil. stainless steel wire of approximately 200 meshes to the inch, the screens being mounted approximately 0.001 to 0.020 inch apart. The degree of capacitance aimed at is generally conventional, but not necessarily so.

In the drawings:

Figure 1 is an end view of two bare wires of the rear screen. Their size is exaggerated. Figure 2 is a corresponding view of the same two wires coated on their upper surfaces with a non-volatile compound of high dielectric strength. Figure 3 is a corresponding view of the same two wires as shown in Figure 2 with the addition of a backing of dried collodion or similar easily sublimated coating composition. Figure 4 in its lower part is a corresponding view of the same two wires shown in Figure 3 plus a continuous metal coating connecting them between the coatings of high dielectric strength compound to close that particular mesh of the screen bounded by the two wires. In addition, two wires above the lower two are shown in corresponding view. These wires are the bare metallic wires of the capacitance screen. Figure 5 corresponds to Figure 4 except that the collodion has been removed by evaporation or solution, leaving a thin metallic sheet connecting the lower two wires, thereby presenting only metal to the electron flow. Figure 6 is a front elevation of the lower screen alone; the capacitance grid is not shown in order to make the view clearer.

In the first five figures the left hand circle is the end of a wire 10 which, cooperating with a right hand wire 11, establishes the width of a mesh space 12 which must'be filled in so that a target area may be formed. The top portion of the wires 10 and 11 are presented in the direction from which the stream of activating electrons is to come. These portions are now coated with an insulating compound of low vapor pressure at ordinary temperatures, namely calcium fluoride CaF2. This coating 13 may be applied by vapor depositing or in the form of a fine water slurry by spraying, painting or rolling on, after the general usage of applying paint. Any known material of the stability and insulating properties of CaF2 may be substituted. Just enough is applied to confer a high resistance to the bounded space 12. High temperature evaporation techniques may also be employed to deposit the coating.

After the CaF2 coating is thoroughly dry, the screen 14 shown in Figure 6, of which the wires 10 and 11 are members, is sprayed or painted from the rear with a collodion or celluloid solution of sutficient viscosity to be able to bridge the space 12 after a short period of application and drying, a rear coating 15 for the screen 14 is thus formed, substantially without coating the CaF2. From the front of the screen 14, then, so as to strike the coatings 13, there is sprayed or evaporated a coating 16 of a highly conductive, non-corrodible metal. Gold is preferred, since it has the added desirability of being ductile and malleable. We have, then, an extremely thin sheet of gold supported on substantially a non-conductor, which divides the sheet into nearly discrete squares. Each square is quite well suited to retain an electrostatic charge for a considerable interval. This interval, while not indefinite, is still substantially longer than those associated with conventional target plates under comparable conditions. Immediate drainage does not take place, partially because all of the charges have the same sign. Aluminum or silver may be substituted for gold.

In use, a screen 18, preferably of identical mesh and wire size is placed before the target plate 14 and suitably charged to direct the electron stream to the center of each metallic square 17 and to give a necessary capacitance efiect. It is also used in the forming of the conducting squares forming a shield whose shadow creates the insulating grid on the target plate.

Since the screen becomes quite warm in use, and a vacuum must be maintained in the tube while the latter is in use, it is important that the collodion backing 15 be completely removed. This is most conveniently accomplished by immersing the target plate in a stream of a solvent for nitrocellulose, for example, acetone, or by Baking at a temperature capable of vaporizing the ma, I

terial away.

Referring now to Figure 6, the wires 10 and 11 are crossed by wires 10a and llato completethe squares. The mesh size and totalfa'ie'a of tlie screen, asfwell} as'tlie wire diameter, is optionalfaccording'to ,the kind, of duty to hedemanded of the 't'ulie inwliich it is to be mounted. The two screens 1'4:andI8j may he'of an optional disfaiice of separation and' bothmay. be mounted together inl'a single frame (not shown) which, ofcourse, must be of a high dielectric character, at least as a portion thereof, so thatelectricalseparation can be maintained between the two screens.

From the foregoing description, it is evident thatlarnew anduseful' process for making target plates has been disclosed; in this processjthe steps are:

11. Coating one side of the wires of a screen with Ca'Fz or equivalent.

2'. Baking the screen with a film of collodion or equivalent.

3. Forming a conductive metallic coating, over the 'CaFz and collodion in such a way as to cover the area between the wires. I

4'. Dissolving or. otherwise removing the collodion.

We claim as our invention: 7

l. The process of preparing a target platefor Orthicon tubes and other electronic devices which comprises coating one side of a metallic screen with asl'urry of a heat- Iiqiiid vehicle, evaporating the vehicle, coating the untreated side of the screen with a solution of an easily soluble lacquer to close the screen'meshes, depositing through a second screen a thin film of a highly electrically conductive malleable metal over the heat-resisting, electrically insulating substance and over the lacquer, after the latter has dried to a film, whereby to close completely ;the meshes ofthe screen while preserving uncoated the heat-resisting, electrically insulating substance, and then removing-the lacquer. p

2. The process of preparing a target screen for Orthicon tubes and other electronic devices, which comprises coating the wires of one side of a stainless steel screen with a slurry of CaFa inwater, drying the so treated screen, coating the back surface of the screen with collodion solution to close the meshes thereof, depositing a thin film of gold over that side of the screenon which the CaFz. is, deposited through a second screen so as to close the meshes of the first screen while preserving the CaFz uncoated immediately under the Wires of the. second screen and then completely removing the collodion.

References Cited in the file of this patent UNITED STATES PATENTS 

1. THE PROCESS OF PREPARING A TARGET PLATE FOR ORTHICON TUBES AND OTHER ELECTRONIC DEVICES WHICH COMPRISES COATING ONE SIDE OF A METALLIC SCREEN WITH A SLURRY OF A HEATRESISTING ELECTRICALLY INSULATING SUBSTANCE IN A VOLATILE LIQUID VEHICLE, EVAPORATING THE VEHICLE, COATING THE UNTREATED SIDE OF THE SCREEN WITH A SOLUTION OF AN EASILY SOLUBLE LACUER TO CLOSE THE SCREEN MESHES DEPOSITING THROUGH A SECOND SCREEN A THIN FILM OF A HIGHLY ELECTRICALLY CONDUCTIVE MALLEABLE METAL OVER THE HEAT-RESISTING, ELECTRICALLY INSULATING SUBSTANCE AND OVER THE LACQUER, AFTER THE LATTER HAS A DRIED TO A FILM, WHEREBY TO CLOSE COMPLETELY THE MESHES OF THE SCREEN WHILE PRESERVING UNCOATED THE HEAT-RESISTING ELECTRICALLY INSULATING SUBSTANCE, AND THEN REMOVING THE LACQUER.. 